Echo returned to a user of a telephone causes the user to hear his or her own voice when speaking on the telephone. In high delay connections, such as those found in satellite or digital cellular applications for example, this echo can make conversation very difficult. To avoid this, it is necessary to ensure that digital telephone terminals provide a significant amount of inherent echo suppression. The amount of echo suppression in decibels that a telephone set provides is referred to as the "terminal coupling loss", or TCL.
As an example, GSM 03.50, an existing specification for digital cellular systems requires a minimum TCL of 46 dB to eliminate the effects of echo. Typically, handset terminals meet this requirement. However, because of the higher electronic gain needed to obtain comfortable listening levels, conventional handsfree terminals fall well short of this requirement. For two-wire analog handsfree terminals, this is normally not a problem because the PSTN (public switched telephone network) provides echo-suppression for long-delay connections. However, in all digital networks, echo suppression is not provided, and hence the task of providing the 46 dB of TCL falls to the telephone terminal itself. The problem is more severe in handsfree digital terminals verses handset terminals because a more significant acoustic echo path exists between the speaker and microphone. In addition, delays are larger in all digital networks and particularly in digital wireless networks. Four-wire analog terminals may also fail to provide sufficient TCL.